1. Field of the Invention
The subject invention relates to a method of producing a uretonimine-modified isocyanate composition having increased low-temperature tolerance and stability.
2. Description of the Related Art
Polyisocyanate compositions including a high concentration of diphenylmethane diisocyanate (“MDI”), particularly 4,4′-MDI are useful for various cellular and non-cellular polyurethane applications. However, a high concentration of 4,4′-MDI often poses a unique processing problem because 4,4′-MDI is normally a solid material at room temperature, i.e., about 25° C. Therefore, the material has to be melted and maintained in order to be useful as a liquid and used in the cellular and non-cellular polyurethane applications.
Unfortunately, MDI compositions having relatively high levels of 4,4′-MDI are also known to have a limited shelf-life due to the formation of diphenylmethane uretdione, otherwise referred to herein as uretdione. Over time, uretdione continues to form in MDI compositions containing 4,4′-MDI. Since uretdione has limited solubility in most MDI compositions, it will tend to precipitate out as a solid. For example, pure 4,4′-MDI compositions maintained at about 43° C. for 14 days have exhibited uretdione concentrations above the generally acceptable saturation concentration of about 0.45%, resulting in the precipitation of uretdione from solution as insoluble white solids. The formation of high concentrations of uretdione renders the MDI compositions substantially useless in many cellular and non-cellular polyurethane applications.
The substantially insoluble uretdione precipitate also causes problems with processing equipment. Specifically, the precipitate clogs the processing equipment, which requires the processing to be stopped. The processing equipment must then be removed from service and cleaned. Even after the processing equipment is cleaned, the precipitate will eventually form and continue to clog the processing equipment resulting in additional service being required. Alternatively, the precipitate may be filtered from the composition. However, additional filtering equipment is required and the removal of the precipitate remains an ongoing problem.
Interestingly, the uretdione formation reaction is both temperature and phase dependent. For example, as the temperature of a 4,4′-MDI composition is increased above about 43° C., the rate of uretdione formation increases. Furthermore, the rate of uretdione formation is accelerated when the MDI is in the solid state as compared with a liquid composition at 43° C., such formation being generally attributed to the alignment of the isocyanate groups in the crystal lattice structure. When the temperature of liquid 4,4′-MDI is lowered and approaches the melting point of about 39 to 40° C., solid 4,4′-MDI begins to form. Along with the formation of solid 4,4′-MDI, insoluble uretdione precipitate rapidly forms, such that even when the temperature is subsequently raised above 43° C., solid particles of uretdione may remain, rendering the product useless for most cellular and non-cellular polyurethane applications.
In view of the temperature and phase dependency of uretdione formation, compositions containing high amounts of 4,4′-MDI are maintained in a liquid state having no solids at a temperature of about 45° C. to minimize the formation of uretdione. However, this desired solids-free liquid state is difficult and costly to maintain and while this temporarily delays the appearance of insoluble precipitate; the uretdione is still being formed at a certain rate. Eventually the concentration of uretdione exceeds the saturation point and insoluble uretdione solids precipitate out of solution.